1. Field of the Invention
The present invention relates to an image stabilizing optical lens device for use in an image-taking optical system and having a so-called anti-vibration function which stabilizes an image against vibration of the image-taking optical system. More particularly, the present invention relates to an optical system having an anti-vibration function of inner focus type, which is capable of preventing degradation of optical performance which otherwise would be caused when the anti-vibration effect is produced by moving an anti-vibration movable lens unit in a direction which is, for example, perpendicular to the optical axis.
2. Description of the Related Art
Image pickup by an image pickup device such as a camera on a running vehicle such as a motorcar or an aircraft tends to suffer from blur of the image due to vibration transmitted from the vehicle to an image pickup optical system, such as a photographing lens of the image pickup device.
It is difficult to stabilize the image against vibration of the image pickup optical system, particularly when the image pickup optical system has a long focal length. More specifically, when the image pickup optical system is tilted due to vibration, the picked-up image vibrates at an amplitude which corresponds to the product of the angle of tilt and the focal length of the image pickup optical system. Therefore, a still image pickup apparatus is required to employ an exposure time which is short enough to avoid degradation of the image quality which otherwise would be caused due to vibration, whereas a dynamic image pickup apparatus suffers a problem of difficulty in maintenance of the image composition. In picking up images with these apparatuses, therefore, it is necessary to effect compensation for tilting of the image pickup optical system caused by vibration, thereby preventing vibratory displacement of the picked-up image, i.e., blur of the image.
Anti-vibration optical systems having a function of preventing blur of a picked-up image are disclosed in, for example, Japanese Patent Laid-Open No. 50-80147, Japanese Patent Publication No. 56-21133 and Japanese Patent Laid-Open No. 61-223819.
More specifically, Japanese Patent Laid-Open No. 50-80147 discloses a zoom lens device incorporating a pair of afocal variable power systems including a first system having an angular magnification M1 and a second system having an angular magnification M2. Magnifying or power varying operations are performed by the first and second focal variable power systems in such a manner as to meet the condition of M1=1xe2x88x921/M2, while the second variable power system is spatially fixed, thereby effecting compensation for vibration of an image, and thus stabilizing the picked-up image.
Japanese Patent Publication No. 56-21133 discloses a system in which vibration of an optical system is detected by a detecting means, and a part of the optical components is moved in response to an output from the detecting means in a direction for canceling vibratory displacement of an image, thereby stabilizing the image.
Japanese Patent Laid-Open No. 61-223819 discloses an image pickup optical system in which a refractive variable-apex-angle prism is disposed closest to the object, the apex angle of the prism being varied to deflect the image in such a manner as to compensate for vibration of the image pickup optical system, thereby stabilizing the image.
In apparatuses of the type disclosed in Japanese Patent Publication Nos. 56-34847, 57-7414 and so forth, an image pickup optical system is provided in a portion thereof with an optical element which is spatially fixed against vibration of the image pickup optical system. The optical element produces a prism effect in accordance with the vibration of the optical system so as to deflect the picked-up image, whereby a still image is obtained on the focal plane.
A method also has been proposed in which a lens unit in an image pickup optical system is vibrated in the direction perpendicular to the optical system in response to an output from an acceleration sensor which senses vibration of the image pickup optical system, thereby stabilizing the picked-up image. This method is proposed, for example, in the specifications of U.S. Pat. Nos. 5,000,549 and 4,974,950, as well as in U.S. patent application Ser. No. 425,749 filed Apr. 2, 1995.
In general, image stabilizing devices of the type which produces a stable still image by vibrating a lens unit so as to cancel the effect of vibration of the optical system are required to provide a large amplitude of compensation for image vibration with a small amplitude or amount of vibratory movement or rotation of the lens unit, i.e., a movable lens unit having plural lenses, which is vibrated to cancel the effect of vibration of the optical system.
Decentering of a movable lens unit causes a blur of the image due to eccentric aberration accompanying compensation for image vibration, when a large aberration occurs, such as eccentric comatic aberration, eccentric astigmatism, eccentric chromatic aberration or eccentric field curvature aberration. For instance, occurrence of a large eccentric distortion causes a large difference between the amount of movement of the image on the optical axis and the amount of movement of the image in the peripheral region. Therefore, when the movable lens unit is decentered in such a manner as to compensate for vibration of the image on the optical axis, a phenomenon resembling the blur of the image takes place in the peripheral region, so as to seriously impair the optical characteristics.
Thus, an optical system having an anti-vibration function, incorporating a movable lens unit movable perpendicularly to the optical axis with or without simultaneous slight rotation about a point on the optical axis so as to cause decentering, has to meet the following requirements.
The first requirement is that the amount of eccentricity aberration caused when the movable lens unit is moved perpendicularly to the optical axis with or without simultaneous rotation, is small enough to suppress degradation of the image quality attributable to such aberration.
The second requirement is that the optical system has a large decentering sensitivity, which is defined as the ratio xcex94x/xcex94H of the amount xcex94x of correction of image vibration to the unit amount xcex94H of decentering movement. This requirement is in order that a large effect of image vibration prevention is obtained with a small amount of movement or rotation of the movable lens unit, thus contributing to a reduction in the size of the whole apparatus.
The optical system having an anti-vibration function relying on spatial fixing of an optical element cannot suitably be used in small and light-weight apparatuses, because of difficulty encountered in the spatial fixing of the element and in the design of a small-sized optical system.
The optical system which employs a variable-apex-angle prism disposed closest to the object also is disadvantageous in that it requires a large-size actuator and in that it is not easy to simply correct eccentric chromatic aberration, although it provides an advantage that almost no aberration other than eccentric (decentering) chromatic aberration is caused.
The optical system of the type which relies on decentering of a lens unit can have a reduced size, provided that the lens unit to be decentered is suitably selected and positioned. This type of optical system, however, suffers from a problem in that it is not easy to achieve a large amount of compensation against vibration with a small amount of actuation while satisfactorily correcting aberrations caused by the decentering, such as eccentricity coma aberration, eccentricity astigmatism, eccentricity field curvature and so forth.
An object of the present invention is to provide an optical system having an anti-vibration function of the type in which a selected lens unit is moved in the direction perpendicular to the optical axis so as to compensate for image vibration, thereby stabilizing the image, wherein the lens elements of the lens unit are suitably arranged so as to enable satisfactory correction of various kinds of aberration, while realizing a large amount of compensation for image vibration with a small amount of decentering, thus contributing to reduction in the size of the whole apparatus. Such an optical system is particularly suitable for use as an inner-focus-type medium-telephoto optical system.
To this end, in one aspect, the present invention relates to an image stabilizing optical lens device, comprising, in order from an object side to an image side of the image stabilizing optical lens device, a first lens unit having positive refractive power, a second lens unit having negative refractive power and being movable along an optical axis of the image stabilizing optical lens device to perform focusing, and a third lens unit having positive refractive power and including, in order from the object side to the image side, a front lens sub-unit having negative refractive power and being movable so as to be decentered with respect to the optical axis, and a rear lens sub-unit having positive refractive power.
In this image stabilizing optical lens device, the focal distances of the lens units are determined in relation to the overall focal length of the whole image stabilizing optical lens device, in such a manner as to meet the conditions of the following equations (1) to (3) in which fi indicates the focal length of the i-th lens unit, and f represents the overall focal length of the whole image stabilizing optical lens device.
0.4 less than f1/f less than 0.8xe2x80x83xe2x80x83(1) 
0.4 less than |f2/f| less than 0.8xe2x80x83xe2x80x83(2) 
xe2x80x830.8 less than f3/f less than 5.0xe2x80x83xe2x80x83(3)
In another aspect, the present invention relates to an image stabilizing optical apparatus comprising, in order from an object side to an image side of the image stabilizing optical lens device, a first lens unit having positive refractive power, a second lens unit having negative refractive power and being movable along an optical axis of the image stabilizing optical lens device to perform focusing, and a third lens unit having positive refractive power and including, in order from the object side to the image side, a front lens sub-unit having negative refractive power and being movable so as to be decentered with respect to the optical axis, and a rear lens sub-unit having positive refractive power, wherein the front lens sub-unit is movable so as to be decentered away from the optical axis in one of (i) a direction perpendicular to the optical axis of the image stabilizing optical apparatus and (ii) a direction along a spherical path defined by a point on the optical axis disposed a predetermined distance towards the image side away from the third lens unit.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments when the same is read in conjunction with the accompanying drawings.